This invention relates to a method of oxidizing organic thiol compounds to disulfide compounds by the use of xanthides.
Thiols have been oxidized by biological oxidants such as flavins, cytochroms, and dehydroascorbic acid [G. E. Woodward, Biochem. J. 27: 1411 (1933)]; by inorganic chemicals such as iodine, hydrogen peroxide, potassium ferricyanide, and nitric acid (Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, Vol. 20, Interscience Publishers, New York, New York, 1969, pages 208-209); and by radiation using X-rays, .beta.-rays , and .gamma.-rays (P.C. Jocelyn, Biochemistry of the SH Group, Academic Press, 1972, page 102). However, most of these reagents are also capable of reacting with other oxidizable sites such as aldehyde and amino groups.
Thiols may also be oxidized by air, but considerable time is required and conditions vary for each thiol. Dimethyl sulfoxide has been used to convert thiols to disulfide, but this procedure requires heating the solution for 6-8 hr. at 60.degree.-80.degree. C. and the removal of byproducts and unreacted DMSO for isolation of the disulfide [C. N. Yiannios and J. V. Karabinos, J. Org. Chem. 28: 3246(1963)].
Organic sulfenyl chlorides have been reductively coupled to form symmetrical disulfides and sulfur-containing polymers [Kobayashi et al., J. Polym. Sci. 10: 3317-3327 (1972)].
We have found a new method for preparing disulfide compounds which comprises reacting a xanthide with an organic thiol, at least one of which is in solution in a suitable solvent, in the presence of an amount of tertiary amine sufficient to initiate the reaction, xanthide and organic thiol being present in a 1:2 molar ratio, respectively.
This method has the advantage of being easy, quick, and general. The reaction is easily taylored so that the byproducts are either volatile or insoluble. Thus, recovery of the essentially pure disulfide compound is simply a matter of filtration or drying. When the reaction utilizes insoluble xanthides in a bed or column, the method becomes a continuous process. Organic disulfide products are used in a variety of ways. Tetramethyl- and tetraethylthiuran disulfides and dimorpholine disulfide aid in the vulcanization of rubber. Cellulose disulfides, such as in cotton fibers which are crosslinked, give crease resistance to the material, However, aside from product use, the method itself is useful. Since the reaction is quantitative and applicable to essentially all thiol compounds, the method is useful in the analytical determination of thiols. Iodine oxidation is the usual analytical procedure for this determination, but iodine is reactive with many other functional groups including multiple bonds. The method can also be used in the petroleum industry where petroleum is "sweetened" by the oxidation of thiol to disulfide.